In addition to its unique genetic features, Prader-Willi syndrome (PWS) is associated with a distinctive behavioral phenotype. Hyperphagia is invariably seen, as are externalizing problems such as temper tantrums and aggression, and a host of compulsive-like behaviors that place affected individuals at high risk for obsessive compulsive disorder (OCD). All these problems are, on average, more frequent and severe than others with mental retardation, and they typically impede the adaptive functioning and life success of persons with PWS. This continuation builds on our previous work by examining both environmental and biological moderators of psychopathological outcome in children and adolescents with PWS. In Specific Aim 1, we clarify the factor structure of compulsive-like symptoms in PWS and also provide the only quantitative assessment to date of hyperphagic symptoms in this disorder. These factor analytic studies assess compulsive-like behavior and food symptoms one time only in 150 participants with PWS aged 4 to 14 years. While Specific Aim 1 fine-tunes outcome variables, Specific Aims 2 and 3 form the centerpiece of the grant, and examine how family and child variables moderate psychopathological outcome. In Specific Aim 2 we assess how family stress, maternal coping, and specific interactions between mothers and children moderate different factors of compulsivity, hyperphagia and other problems (and also how child problems differentially moderate family functioning). In Specific Aim 3 we examine how such child factors as age, genetic subtypes of PWS (paternal deletion of 15ql I-q13 versus maternal uniparental disomy), and serotonergic functioning are associated with compulsivity, hyperphagia and other problems. These two Aims, then, begin to explain individual differences in the behavioral phenotype of PWS, and ultimately, why some individuals have more successful life outcomes than others. To this aim, the continuation uses a within-syndrome, longitudinal design with 40 persons aged 4 to 14 years who are assessed at several time points over a 5-year period. We have already followed approximately 60% of these 40 participants, providing a firm foundation for the continuation. The 4 to 14 year age period is one when compulsive-like, hyperphagic, and other behavioral problems are increasingly seen, and as such we can track both the emergence and correlates of these problems. As participants will be assessed from 3 to 4 times, individual growth curve modeling techniques will be used to identify the trajectory and correlates of compulsivity, hyperphagia, and other problems. Findings will further delineate the PWS phenotype across genetic subtypes, and for the first time ever, elucidate both environmental and biological correlates of psychopathology in this syndrome.